Embryos (zygotes) are produced by in vitro fertilization of sperm and oocytes in a culture system. The embryos are then cultured to a stage of hatched blastocysts having hatched from the zona pellucida via the stages of cleavage, morula, and then blastocyst. As a result of realization of these techniques, assisted reproductive technology (ART) has been established not only in the field of domestic animals, but also in the field of medical care for human infertility, which involves implantation of the embryos at a stage between cleavage and blastocyst into the uterus, no as to obtain infants.
However, in vitro fertilization does not always result in a high pregnancy success rate. For example, the pregnancy success rate of human in vitro fertilization still remains at a level of about 25% and 35%. A reason for this is the low possibility of obtaining high-quality embryos suitable for implantation into the uterus by culture. Whether or not cultured embryos are high-quality embryos suitable for implantation into the uterus is identified by specialists who observe each embryo via a microscope.
In in vitro fertilization, a microdrop method is often used where droplets of a culture solution are produced in a vessel, and embryos are put in the vessel to effect in vitro culture. Conventionally, in the microdrop method, a petri dish having a bottom surface with a single plane and with a diameter of 30 to 60 mm is used as a cull culture vessel, and a plurality of droplets of a culture solution are produced at intervals on the bottom surface of the petri dish, so that cells are cultured in the droplets.
When droplets are produced in an ordinary petri dish, the position of each embryo may change due to the cell movement of the embryo itself or due to convection that may occur within each droplet. Thus, it becomes difficult to identify embryos that have been cultured and observed in the petri dish, which is problematic. Thus, a means capable of controlling the positions of embryos has been demanded.
In order to increase the effect of culturing embryos more efficiently, it is preferable to use the interaction (i.e., paracrine effect) between embryos. In order to control the positions of embryos while using such effect, there is known a system for culturing embryos in which microwells with about the same size as embryos are formed at the bottom of a petri dish so that droplets of a culture solution are added covering the plurality of microwells, and then, the embryos are placed in the microwells filled with the culture solution. Accordingly, it becomes possible to individually observe a plurality of embryos while controlling the positions thereof, and culture a plurality of embryos in a small amount of culture solution while using the paracrine effect.
Meanwhile, in order to distinguish between individual embryos, it is necessary to identify individual microwells. Since microwells are observed with a microscope, it is necessary to estimate as many pieces of information as possible only from the field of the microscope. However, if the magnification is high, it is impossible to determine which microwell is currently viewed. Thus, in order to identify individual micro-wells, there is known a method of adding information, such as numbers or characters, to the outer periphery of a microwell array so that the microwells can be identified in a matrix manner.
However, when a microwell is observed with a microscope at high magnification using the above method, it is necessary to read the identification information by greatly shifting the observation position from the current microwell, which is problematic in terms of workability. Further, when a cell is imaged with a microscope, it is necessary to manually provide information to the obtained photograph data, which involves complex work, as the photograph does not originally contain the identification information on the microwell. Furthermore, there is a possibility that the operator may make a mistake in the process of associating the information with the photograph data.